At present, there are rapidly growing demands for an increase in the capacity and speed of optical communication systems and for improvements in the functions of these systems. Optical switches and optical modulators are expected to serve as optical signal processing devices used for the optical communication system. In particular, optical crossconnect switches are becoming increasingly important in meeting the recent demands for improvements in the functions of networks. As such optical switches, the following have been developed: optical switches called MEMS (Micro Electromechanical System) and using a micromachine technique, and optical switches using the thermophotometric effect of silica on silicon waveguides. Further; optical switches have also been developed in which crossing parts of waveguides are filled with oil having a refractive index equal to that of the waveguides so that the waveguides are overheated to generate bubbles to reflect light at the crossing parts, thus switching an optical path.
However, these optical switches operate at a speed on the order of msec. An operating speed required by next-generation optical networks to route optical packets is 1 to 10 nsec. Optical switches using the electro-optic effects of LiNbO3 (also referred to as LN) can accomplish an operating speed of 1 to 10 nsec. These switches are provided by varying the refractive index of waveguides on the basis of the primary electro-optic effect of the LN.
As a method for producing an optical switch based on the electro-optic effect of the LN, one is known which comprises using a Ti heat diffusion process to form a waveguide pattern having a Y branch form, on an LN substrate, then forming a buffer layer on the waveguide pattern, and further disposing electrodes corresponding to the waveguide pattern, on the buffer layer (refer to Nishihara, Haruna, and Suhara, “Optical Integrated Circuit”, OHM Co., Ltd., pp. 310 to 326 (1985)).
However, the LN is a trigonal crystal, and light must guided in an orientation with birefringence in order to use r33, which is a larger electro-optic constant. Thus, the optical switch has a polarization dependency, that is, its operation varies with the polarization of light. The polarization dependency may cause an error in the transmission of light. Accordingly, it is important that the optical switch is a polarization independence. Thus, attempts have been made to produce optical switches operating at the polarization independence, using the LN. However, with a crystal orientation without birefringence, r13, which has a smaller electro-optic constant, is utilized, thus resulting in a driving voltage of 40 V or more.
As an optical switch operating at high speed, one has been proposed which uses a Mach-Zehnder interferometer composed of a semiconductor material. However, the Mach-Zehnder interferometer uses control light for switching and requires complicated arrangements for synchronization and the like. Consequently, the Mach-Zehnder interferometer is not practical.
Further, adigital optical switch has been proposed which controls a mode distribution. However, such a switch uses a higher driving voltage than other types of switches.
By the way, the electro-optic effect is a phenomenon in which the electron state of atoms constituting crystal is varied by electric fields. Accordingly, this effect responds very quickly to a change in electric fields. Thus, the electro-optic effect responds instantaneously to a change in electric fields on a femto second level. Consequently, this effect is a physical phenomenon optimum for very fast optical modulators. In particular, the LN can be relatively easily formed into optical waveguides using a method such as diffusion of impurities such as Ti, or ion exchange. Accordingly, the LN is a material also widely used for optical modulators utilizing the primary electro-optic effect (refer to Nishihara, Haruna, and Suhara, “Optical Integrated Circuit”, OHM Co., Ltd., pp. 310 to 326 (1985)), Japanese Patent Application Laid-open No. 53-006054 (1978), and Japanese Patent Application Laid-open No. 53-054040 (1978).
In general, the electro-optic effect is dependent on the orientation of a crystal; electric fields are applied to a crystal axis having the largest electro-optic constant to modulate the refractive index. The LN utilizes the r33 (30 pm/V), described above. Further, the operating speed and modulation voltage are important performance parameters for implementation of an optical modulator. The magnitude of phase modulation is proportional to the length of electrodes. Accordingly, a required modulation voltage decreases consistently with increasing electrode length. However, when the electrode length is 1 cm or more, it is difficult to uniformly apply a high frequency on the order of GHz using lumped constant electrodes. This is because the period of a modulation signal is equivalent to the time required by electric fields to move from end to end of an electrode. In contrast, if the electrode length is reduced in order to improve a response speed, a high voltage power source is required. Thus, in this case, an actually available power source is very expensive.
It is an object of the present invention to provide an optical switch, an optical modulator, and a wavelength variable filter each of which has a simple configuration, which requires only a low driving voltage, which is independent of polarization, and which can operate at high speed.